FIG. 1 shows a cam summation engine valve system as disclosed in U.S. Pat. No. 6,941,910. The valve system includes two cams 10 and 12 and a cam summation rocker 14, herein also termed an “upper rocker,” having cam followers 16 and 18 in contact with both cams. A lower actuating rocker 20 is pivotably connected to the summation rocker 14 and acts, at one end, on a valve 22, with its other end resting on a hydraulic lash adjuster 24. An adjustable stop plate 26 is used to limit the expansion of the hydraulic lash adjuster 24 by setting the height of the pivot shaft 30 that connects the lower rocker 20 to the upper rocker 14. The position of the lower rocker 20 is therefore defined by its contact with the tip of the valve 22, and the expansion of the hydraulic lash adjuster 24 holding the pivot shaft 30 against the adjustable stop plate 26.
Cam summation valve systems using hydraulic lash adjusters have required an adjustable stop, or a graded shim in order for the system clearance (and hence the valve lift) to be adjusted. The functions of this clearance adjustment are twofold. First, the expansion of the hydraulic lash adjusters is limited so that the correct amount of clearance is maintained in the system while the valves are closed. Second, the valve actuating rocker is held in contact with the tip of the valve by the expansion of the hydraulic lash adjusters and the clearance adjustment system so that any clearance must occur between one of the cam profiles and its respective follower(s).
G.B. Patent Application No. 0708967.5 (WO2008/139221), by the instant Applicants, describes a cam summation engine valve system as shown in FIG. 2. This figure shows a similar valve system to that shown in FIG. 1. In both drawings, like parts have been allocated the same reference numerals to avoid repetition. In FIG. 2, the valve actuating lower rocker 20 is mounted on a manually-adjustable pivot 32. The valve lift is adjustable through a screw mechanism 34 and contact is maintained between the tip of the valve 22 and the lower rocker 20 at all times through a control spring 36.
This configuration replaces the hydraulic-lash-adjusting elements with a mechanical clearance adjustment and maintains the correct amount of clearance in the system while the valves are closed. However, an adjustable pivot 32 is required to allow the amount of clearance in the system to be adjusted. In the absence of such adjustability, there would be no way to compensate for manufacturing tolerances, which may lead to significant variations in valve lift between cylinders, and potentially damaging impact forces between the components of the system. While it provides for clearance adjustment, the system of G.B. Patent Application No. 0708967.5 requires a significant amount of packaging space that may not be available in all engines.
Therefore, a need exists to overcome the problems with the prior art as discussed above.